AlphaGo introduce a new approach to computer Go that uses ‘value networks’ to evaluate board positions and ‘policy networks’ to select moves. These deep neural networks are trained by a novel combination of supervised learning from human expert games, and reinforcement learning from games of self-play.

Without any lookahead search, the neural networks play Go at the level of state-of-the-art Monte Carlo tree search programs that simulate thousands of random games of self-play. AlphaGo also introduce a new search algorithm that combines Monte Carlo simulation with value and policy networks.

Read more:

https://googleblog.blogspot.com.au/2016/01/alphago-machine-learning-game-go.html
http://www.nature.com/nature/journal/v529/n7587/full/nature16961.html